The Earth itself comprises a biosphere in which microorganisms, plants, and animals, including humans, exist in a more or less steady state, wherein matter is a finite resource which is continually recycled. There is continual energy input in the form of solar radiation. The quantity of matter gained or lost to space outside the Earth's atmosphere is minute. Thus, the Earth is a closed ecological system which may be referred to herein as Biosphere I.
It is desirable to provide a microcosm of the biosphere known as Earth for study of the interaction of components, and development of techniques for influencing our environment. Such experiments are difficult at best in the open system provided on Earth, where matter is exchanged between the Earth's environment and the experiment itself. It is, therefore, desirable to provide a system that is completely enclosed so that no matter is exchanged with the Earth's environment.
There is, therefore, being established near Oracle, Ariz., a completely closed ecological system referred to as Biosphere II. The system completely encloses about one hectare of land and 142,000 cubic meters of space isolated from the Earth's environment by an impermeable skin so that no matter is transferred. The above ground portion of the skin is transparent glass for receiving solar radiation. Electrical energy is provided to the closed system, and heat may be transferred to or from the system as required. Thus, the Biosphere II closed ecological system is closed as to matter, but open as to energy. For meaningful research, it also remains open for transfer of information.
The closed system should maintain an atmosphere with a composition suitable as a habitat for humans and other animals. For example, it is important to maintain the carbon dioxide concentration in the atmosphere within the closed ecological system within limits that are tolerable by the human and other animal occupants of the system, and provide for a desired steady state growth of plants for providing edible biomass and recyclable carbonaceous materials. To study the effects of various factors on the environment in the system it is of great importance that the impermeable skin of Biosphere II remain substantially completely sealed so that there is virtually no exchange of air between the inside and outside of the system.
The Biosphere II closed system has a large steel space frame erected on a concrete foundation. The space frame serves as support for a safety glass glazing system which provides the impermeable skin for the system. While glazing systems for use with space frame structures are known, a need exists for a glazing system providing an environmentally isolated enclosure which is virtually free of any gas leakage.
It is desired that the glazing system have a leakage rate not exceeding 1% per year of the enclosed volume of air for a period of many years. Considering the size of the structure and the miles of seals along the edges of glazing panels, this is a formidable challenge. An effective system should comprise panels and attachment elements capable of being mounted easily, quickly and safely to space frame structures and involve uncomplicated mounting structures and procedures. An effective system should also provide means to enable the detection of air seepage through the seals in the event it should occur.
The basic problem to be solved is to achieve air-tight seals between adjacent materials which may move with respect to each other. Such movement is due to thermal expansion and contraction, and to structural stresses such as are induced by wind-loading. One approach to solving this problem is to employ a sticky, flexible material to fill the moving joints. It is questionable, however, that available materials will remain flexible and in perfect adherence indefinitely. The best of the materials is probably a silicone resin, because it does not degrade appreciably by weathering or sun-light exposure for very long times, but it is uncertain whether the liquid-applied silicone adhesives can be reliably applied to form and maintain air-tight adhesion.
Glazing systems are customarily designed for preventing water leakage, which is a relatively easier task than preventing gas leakage. Water has surface tension which largely prevents it from entering minute crevices or holes. It also flows downhill while gas can follow any passage, no matter how long or convoluted. Because of such differences, the architectural glazing industry uses designs intended to keep rain out, and has very little experience applicable to the problem of air leakage.